The 12 volt systems used in today's automobiles are required to supply ever increasing currents as the load on the system continues to increase. This increase is due to a combination of increasing numbers of electronic devices, such as communication, entertainment, and telematics systems, as well as the proliferation of electric powered auxiliary systems to replace traditional hydraulic or mechanical powered systems. To reduce the amount of current required to supply these higher loads, it has been proposed that automobiles should adopt 42 volt electrical systems. However, the automotive industry has been reluctant to transition to 42 volt electrical systems because of increased costs. Consequently, there is a strong demand to improve the performance of 12 volt systems, thereby allowing higher electrical loads to operate effectively with conventional vehicle electrical systems.
One principal limitation in the performance of automotive electrical systems is the alternator, and particularly the amount of current that can be drawn from the alternator and the response time required to draw this current. The output current capability of today's automotive alternator is influenced by the speed at which the alternator is operating, which is determined by the engine speed of the vehicle. A typical alternator might produces a rated current of 135 amperes at an engine speed of 3000 rpm, might typically produces only 60 amperes at an engine speed of 600 rpm (corresponding to engine idle). Most automotive electrical loads are insensitive to vehicle speed, such as rear window defoggers, heated seats, lights, HVAC blowers, entertainment devices, etc. The loads that are sensitive to engine speed (e.g., ignition) do not consume significant current. Consequently, the electrical system is in significant current deficit at idle, which can produce voltage fluctuations if additional current is required. Furthermore, because some loads are very sensitive to voltage fluctuations (e.g., lights), an alternator that responds quickly to large load application will reduce the undesirable effects of voltage fluctuations, such as light flickering, seen by the driver.
It bears mentioning that the problem addressed herein is not one of power limitation where the alternator cannot supply sufficient power, but rather is a problem of voltage limitation. Fundamentally, the back emf (electromotive force) produced by the alternator is not large enough to supply the required current at engine idle, even with the field current at its max value.
Solutions to address certain aspects of the performance deficiencies in automotive electrical systems are addressed in co-pending U.S. patent application, having the Ser. No. 10/927,687, filed Aug. 27, 2004, and entitled “Automotive Electrical System Configuration.” This patent application is hereby incorporated by reference in its entirety. The present invention attempts to further minimize the above-mentioned drawbacks and proposes a system that solves or at least minimizes the problems of the prior art.